(1) Field of the Invention
The present invention relates to a liquid crystal element, and a manufacturing method and a driving method thereof.
(2) Description of the Prior Arts
A liquid crystal element is widely utilized for such a monitor and a projection type display as a word processor and a computer, and a small-sized portable television, and such an element for controlling light as an optical switching element. A conception of a liquid crystal element includes a liquid crystal display device and an optical switching element. A liquid crystal display device is described as a liquid crystal element unless mentioned particularly in the specification.
A twisted nematic (TN) liquid crystal and a vertically aligned (VA) liquid crystal are mentioned as a typical example of a liquid crystal used for a liquid crystal element.
A twisted nematic (TN) liquid crystal has a drawback of a low contrast because of an alignment directivity of a liquid crystal molecule near a reverse tilt domain and an interface. Moreover, in both a TN liquid crystal and a VA liquid crystal, light transmittance and reflectance vary greatly with an angle with a liquid crystal element because of the directivity of molecular array, and both liquid crystals have a characteristic of viewing angle whereby the coloring and the decrease of contrast are caused.
In order to correct the viewing angle, a retardation film and a scattering film are disposed on a surface of a liquid crystal element using a TN liquid crystal. Meanwhile, in recent years a method of dividing an alignment direction on a pixel into two or four parts is used for a VA liquid crystal in order to widen the viewing angle, however, the problem is that the utilization efficiency of light decreases and the response time of liquid crystal decreases, and additionally the process of dividing an alignment direction on a pixel and aligning a liquid crystal divisionally becomes complicated. It has been considered that a generally high contrast is obtained in a VA liquid crystal, however, a contrast on a supreme level can not obtained because of light leakage by a slight inclination of a liquid crystal molecule in a black display.
Thus, the problem is that conventional TN liquid crystal and VA liquid crystal have a great dependence on viewing angle, and the coloring, the decrease of contrast and tone reversal occurs according to a direction of viewing. Furthermore, the speed of response is unsatisfactory.
Recently, an in-plane switching (IPS) method, which is developed for improving a characteristic of viewing angle, is noticeable. Although the IPS has a superior characteristic of viewing angle because of driving a liquid crystal molecule on a substrate plane, an insulating layer on an electrode is necessary for preventing a short circuit between electrodes because of forming both electrodes on the same plane of the same substrate. The problem is that the IPS has an image persistence in displaying a fixed pattern for many hours because of the insulating layer.
A general liquid crystal element has a structure in which liquid crystal is sealed between two substrates. When an alignment direction of a liquid crystal molecule in liquid crystal becomes nonuniform in sealing liquid crystal between two substrates, irregularities occur in an alignment direction of a liquid crystal molecule of each pixel on an initial condition. Then, even if a voltage on the same condition is applied to each pixel, a disclination line appears nonuniformly at each pixel. When a display image is a black screen in using a liquid crystal element as a liquid crystal display device, an observer of the screen sees as if white sand would be scattered on the black screen and the screen were spotted.
In a homeotropic type liquid crystal display device (a liquid crystal element) wherein all liquid crystal molecules are aligned vertically to a substrate, a method of improving a display characteristic by providing an aperture for an opposite electrode and controlling electric field is disclosed in Japanese Unexamined Patent Publications No. 3-259121 and No. 6-301036.
The Publication No. 3-259121 relates to a dot matrix type liquid crystal display device, and as shown in FIG. 1, a slender aperture 51 is provided for a crossing portion of a first electrode (pixel electrode) 1 on the side of a TFT substrate and a second electrode (opposite electrode) 3 on the side of an opposite substrate, namely, the second electrode (opposite electrode) 3 in a display portion in a pixel.
Therefore, a diagonal electric field is caused in the crossing portion of the first electrode (pixel electrode) 1 and the second electrode (opposite electrode) 3, and as shown in FIG. 1(b), a liquid crystal molecule 7 is aligned uniformly in a predetermined direction by the diagonal electric field.
The Publication No. 6-301036 relates to an active matrix type liquid crystal display device, and an area, in which a liquid crystal molecule is not inclined remaining vertical, exists by providing an approximately square aperture for a central portion of an opposite electrode, which is opposite to a pixel electrode, namely, a display portion of a pixel, and forming an area with little or no electric field in the pixel electrode despite an existence (ON, OFF) of display. Therefore, the array of a liquid crystal molecule in other areas of the pixel is improved, a disclination line, which is a drawback resulting from the nonuniformity of array, is fixed, spots of the screen never occurs, the liquid crystal molecule is inclined in various directions (four directions) in a pixel, and thereby a characteristic of viewing angle is improved.
However, in either of these methods, since an aperture exists in an opposite electrode, opposite to a pixel electrode, namely, originally a display area, a disclination line always occurs in such a boundary between areas wherein a liquid crystal molecule is inclined in a different direction, and an area wherein a liquid crystal molecule is not inclined despite an existence of electric field as a central portion and a corner of the aperture. FIG. 1(c) shows this state in a liquid crystal display device of the above-mentioned Publication No. 3-259121. As shown in FIG. 1(c), a disclination line 9 occurs at both ends of the aperture 51.
Consequently, a liquid crystal display device (a liquid crystal element) having an aperture causes a remarkable decrease in light transmittance, such as a black portion in a white display.
Since an aperture exists in an opposite electrode in a pixel, which is originally a display portion, the resistance of light transmission increases. In addition, since an aperture which does not conduct electricity exists in an opposite electrode, a voltage in an opposite electrode away from a connection terminal portion decreases greatly, whereby the unevenness of luminance on a display plane occurs.
Accordingly, it has been desirable to actualize a liquid crystal display device wherein, by means of not causing a disclination line in a pixel display portion, a decrease in the utilization efficiency of light is never caused, the unevenness of luminance on a display plane never occurs, and the viewing angle toward up and down as well as right and left is improved.